FIG. 1 shows a structure described in Patent literature 1 as a conventional example of this type of microphone.
In this example, two electret condenser microphone units are disposed in a holder 1. In FIG. 1, the microphone units have diaphragms 2a and 2b, and opposite electrodes (back plates) 3a and 3b are respectively disposed opposite to the diaphragms 2a and 2b. The opposite electrodes 3a and 3b are connected to the gate terminal of a field effect transistor (FET) 4.
The opposite electrodes 3a and 3b and the FET 4 are supported by a supporting member 5, and the opposite electrodes 3a and 3b are disposed opposite each other with the FET 4 placed therebetween. The diaphragms 2a and 2b are positioned at the outer sides of the opposite electrodes 3a and 3b, respectively.
The holder 1 has a through hole 6 and also has a narrow gap 7e between the supporting member 5 and the inner wall of the holder 1. Ring-shaped members 8a and 8b provided at the outer sides of the diaphragms 2a and 2b in order to form outer cavities 7a and 7b are cut to form paths 7c and 7d, respectively.
Sound waves input from the through hole 6 pass through the narrow gap 7e, the paths 7c and 7d, and the outer cavities 7a and 7b to reach the diaphragms 2a and 2b. Independent inner cavities 9a and 9b, not connecting with each other, are formed between the opposite electrodes 3a and 3b. 
With this structure, in-phase output signals can be obtained from the two microphone units for the input sound waves, whereas opposite-phase outputs can be obtained for vibration noise caused by mechanical vibration, allowing the vibration noise to be canceled.